The concentrating solar power industry has a three-decade long performance record, applying various engine cycles such as Stirling, Rankine, and Brayton to the task of converting solar energy into electric power. Stirling engines located on 3-meter to 11-meter diameter parabolic-dish concentrators successfully demonstrated the ability to convert the solar energy to shaft power and in turn produce electric power using an electrical generator. Rankine cycle converters, using steam or alternative organic fluids to drive a turbine, have been installed on similar parabolic concentrators as well as parabolic troughs. The Brayton cycle, or so-called gas turbine, has been installed on parabolic-dish concentrators, typically utilizing a highly modified commercial gas turbine engine with a solar receiver to absorb the concentrated solar energy and heat air. Though none of these apparatuses would be described as commercially available, fundamental thermodynamic of these prime movers has been demonstrated with a solar heat source.
FIG. 1 illustrates generic components of a system designed to convert concentrated solar energy into electricity using a “heat engine” or “prime mover” 13. Using a reflective surface, a solar concentrator 9 focuses solar energy to an intensity of between 100 and 5,000 times that normally incident on Earth. The concentrated solar power is directed on an absorbing material of a receiver built into the prime mover 13, thereby heating a fluid flowing through the solar absorber (the details are not shown). The fluid, known as a “working fluid,” drives a thermodynamic “heat engine” which in turn produces shaft power. The shaft power is converted to electricity using conventional electro-magnetic generator principles.
The Brayton cycle is an example of a cycle that may be used with the power conversion system 13a of FIG. 1. In known types of such “Dish-Brayton” systems, the working fluid is air, and follows a typical path through the engine. FIG. 2 shows air entering the engine through an intake filter 1, and then into a compressor section 20. The fluid exiting the compressor is elevated in pressure to typically 3 to 6 atmospheres. The compressed air then may flow through a recuperator 7 and into the solar receiver 8, or alternatively into the receiver 8 without recuperation. The pressurized air is heated in the solar receiver to a temperature of about 800 to about 1000 degrees Celsius. A combustor, designed to burn conventional fuels, may be used at this point in the cycle to further heat the air, or make up for deficient solar input. At this high pressure, high temperature state, the air expands through a turbine stage 10, consisting of one or more turbine rotors. The turbine stage is connected by a shaft or shafts 109 to the compressor 20 and the electrical generator 21. The Brayton cycle engine operates with a continuous flow of air and heat addition, producing a proportional amount of continuous electrical power.
In summary, the solar-activated “Dish-Brayton” module operates precisely as a conventional gas turbine generation unit, with the added complexity of a component in the cycle to receive solar energy and heat the working fluid.
A related type of power conversion system is referred to as the Compressed Air Energy Storage (CAES) system. The system employs a principle of periodic compression and expansion (e.g., turbine) stages to generate power. A plant, shown schematically in FIG. 3, consists of a compressor station 20 and a turbine station 10. Fuel is burned in a combustor 22 to heat the working fluid. The CAES plant utilizes a compressor station to charge a vessel 6 with pressurized air. The air compressor station 20 would typically employ intercooling to improve efficiency. The air storage vessel may be of conventional steel construction or utilize a specially prepared geological formation. The geological formation resembles the salt domes and aquifers currently used for natural gas storage.
One of the attractions of CAES is electric utility load shifting. Energy is stored at night when utility rates are low. Then, during the day where electric rates are high and loads are generally high, the CAES plant operator may release the air pressure through the turbine-generator to generate power.